samples: adc_dt: Fix adc_dt run issue on NXP MCXN boards

Fix adc_dt run issue on NXP MCXN boards, the default
mode of vref is standby mode, the BUF21 is not enabled,
so the LPADC can not work normally. Need to set the vref
mode to 'NXP_VREF_MODE_LOW_POWER' or 'NXP_VREF_MODE_HIGH_POWER'
to enable the BUF21.

Signed-off-by: Zhaoxiang Jin <Zhaoxiang.Jin_1@nxp.com>